This application is based on the application No. 2002-028300 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor device incorporating a current-sensing element for preventing an over-current and further including an insulated-gate transistor such as an IGBT (Insulated Gate Bipolar Transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) or the like which has a means for protecting the current-sensing device from being destroyed by an over-voltage.
2. Description of the Prior Art
In recent years, an insulated-gate transistor (voltage-driven transistor) such as an IGBT, a MOSFET or the like has been widely used as a switching element etc. such as an inverter, an AC servo or the like, because it can execute a high-speed switching operation. In general, the insulated-gate transistor described above is provided with a current-sensing element in order to prevent that an over-current flows through a circuit, an external load or the like.
To be concrete, for example, in each of the Japanese Laid-Open Patent Publications Nos. 9-293856, 4-326768, 20002-69489, 8-46193 and 6-13618, there has been proposed an IGBT device or MOSFET device, which incorporates a current-sensing element for detecting a current flowing through a principal IGBT or a principal MOSFET to control a principal current, and prevents an over-current by lowering the gate voltage of the principal IGBT or the principal MOSFET when the current sensing element has detected a rise of the current in the principal IGBT or the principal MOSFET.
FIG. 7 is a circuit diagram showing a rough construction of a conventional semiconductor device (IGBT device) of a general type, which incorporates a current-sensing element. As shown in FIG. 7, in the conventional semiconductor device, a first IGBT 101 for controlling the principal current and a second IGBT 102 (current-sensing element) for monitoring the current flowing through the first IGBT 101 are formed together on one semiconductor substrate (not shown). In the semiconductor device, each of a gate 103 and a collector 104 is common with both the IGBTs 101 and 102.
On the other hand, each of the two IGBTs 101 and 102 is provided with a respective emitter. That is, a first emitter 105 is provided for the first IGBT 101 while a second emitter 106 is provided for the second IGBT 102. Thus, both the emitters 105 and 106 are connected to a common emitter 107. Because the second emitter 106 is provided only to monitor the current in the first IGBT 101, its size or area is generally much smaller than that of the first emitter 105.
The second emitter 106 is provided with a sensing resistor 108. Thus, the voltage applied to the gate 103 is controlled in accordance with the sense voltage Vs applied to the sensing resistor 108 (i.e. voltage drop due to the sensing resistor 108).
As shown in FIG. 8, in the conventional semiconductor device, the sense voltage Vs applied to the sensing resistor 108 is approximately proportional to the current flowing through the first emitter 105. Therefore, an over-current in the first IGBT 101 may be prevented by monitoring the sense voltage Vs and feeding back it to the gate 103.
As described above, the size or area of the second emitter 106 is very small while the capacity of the second IGBT 102 used as a current-sensing element is also very small. In consequence, there exists such a problem that the second IGBT 102 or the second emitter 106 is easily affected by an external circuit so that the second IGBT 102 or the second emitter 106 may be destroyed due to an over-voltage or the like according to the situation. As an example of the destruction described above, for example, there may be pointed out such a case that an voltage higher than the voltage of endurance (breakdown voltage) is generated between the first emitter 105 and the second emitter 106.
It goes without saying that the above-mentioned problems may occur also in a power transistor of a voltage-driven type such as a MOSFET other than the IGBT.
The present invention, which has been developed to solve the conventional problems described above, has an object to provide a means for an insulated-gate transistor such as an IGBT, a MOSFET or the like incorporating a current-sensing element for preventing a over-current, which can effectively prevent the over-current and further can effectively prevent destruction of the current-sensing element due to an over-voltage or the like.
A semiconductor device according to the present invention includes a first transistor of an insulated-gate type (e.g. IGBT, MOSFET or the like) and a second transistor of an insulated-gate type (e.g. IGBT, MOSFET or the like). The first transistor controls a principal current while the second transistor monitors a current flowing through the first transistor. Hereupon, the term xe2x80x9cprincipal currentxe2x80x9d means the current flowing through an external load which is controlled by the semiconductor device. Both the transistors are formed in a common semiconductor substrate. In addition, both the transistors shear a common collector region and a common gate region to each other. An emitter region of the second transistor is smaller than an emitter region of the first transistor, as for their sizes or areas.
Further, the semiconductor device includes a sensing resistor disposed between the emitter region of the first transistor and the emitter region of the second transistor. A voltage applied to the common gate region (furthermore, a current flowing through the first transistor) is controlled in accordance with a sense voltage applied to the sensing resistor, namely a voltage drop due to the sensing resistor.
In addition, the semiconductor device includes a diode portion or diode device (i.e. clamping element). The diode portion is provided between the emitter region of the first transistor and the emitter region of the second transistor in parallel with the sensing resistor. Hereupon, the diode portion becomes a breakdown state when a voltage, which is lower than an endurance voltage (i.e. breakdown voltage) between both the emitter regions and is higher than an upper limit of the sense voltage, is applied thereto. In consequence, an over-current of the first transistor may be effectively prevented by monitoring the sense voltage of the second transistor. In addition, the second transistor may be surely prevented from being destroyed by an overvoltage.
Alternatively, the diode portion may become a conductive state when the voltage, which is lower than the endurance voltage and is higher than the upper limit of the sense voltage, is applied thereto. In this case also, the over-current of the first transistor may be effectively prevented, and further the second transistor may be surely prevented from being destroyed.